The present invention relates to sending feedback data to a sender of packets in a wireless communication system and in particular is directed to a new feedback channel for use in fast hybrid automatic repeat request.
Fast hybrid automatic repeat request (HARQ) has been proposed as a vehicle to solve receiver memory problems which occur when soft combining schemes are used in wireless communications. The fast HARQ requires that an acknowledgement (or the so-called forward order) be transmitted within the next radio frame after the transmission of packet(s) in the forward direction. Hybrid ARQ (HARQ) is a link adaption technique used to improve the performance of wireless communications. In a Type 1 Hybrid ARQ, there is soft combining which is a type of repetition coding in which the retransmitted packet is combined with the initially transmitted packet. In Type 2 Hybrid ARQ, an incremental redundancy scheme is used. In both Type 1 and Type 2 Hybrid ARQ, the soft decision values of the erroneous packet, if detected, must be stored in the receiver which in turn can lead to very high memory requirements at the receiver. Thus the amount of memory required for storing these soft decisions is proportional to the retransmission time interval. Fast hybrid ARQ has been proposed to decrease retransmission delay which in turn decreases the memory requirements at the receiver.
In prior art schemes, the whole ARQ protocol is in the radio link control (RLC) layer and the feedback data as well as the retransmissions have been generated in the RLC layer as described in the Release 1999 3GPP specifications. This type of feedback data is not suitable for fast HARQ if the RLC in the network side is located in the radio network controller (RNC) since the Iub interface between RNC and Node B (the base station) can cause long delays. The delay of the Iub interface is one of the main reasons for the long round trip delays. This prior art technique requires that the retransmission delay becomes very high (typically from approximately ten to twenty transmissions in time intervals (TTIs). This retransmission delay therefore implies that the memory requirements at the transmitter, but especially in the receiver, must be very high in order to be able to retransmit and soft combine the retransmitted packets with the stored packets which were received erroneously.
One way to speed up the whole process is to generate the feedback data in the physical layer of the receiver. Similarly, the retransmissions should be generated at the physical layer of the transmitter. Alternatively, the feedback and the retransmission can also be generated in a layer which is co-located with: the physical layer, thereby eliminating any long delay between these two layers.
There are several ways of transmitting the feedback data. One possibility is to transmit it through existing uplink or downlink channel. This has the problem that the existing channels are usually terminated in the radio network controller (RNC) in the network side, i.e., thee is the delay between the base station and the RNC. Even if the termination of the existing transport channel were changed to a base station in the network side, the transmission delay would be at least three TTIs more than that of the proposed invention since the existing transport channels are interleaved at least over ten milliseconds (ms) (see 3GPP spec).
All the previous implies that a separate fast feedback channel needs to be defined. One straightforward possibility in a CDMA system is to transmit the feedback data using a separate code channel and transmit it in parallel with other data which has been proposed for instance by Motorola Corporation for its one XTREME system. This requires multi-code transmission which is not desirable in the mobile terminal (if the feedback is in the uplink direction).
The present invention provides a solution for fast feedback associated with-fast HARQ and thereby solves the buffering problem associated with other feedback mechanisms. In particular, the technique of the present invention does not require the use of a separate code channel for feedback information but rather is able to steal some of the capacity from uplink traffic data or control traffic (or downlink traffic depending upon the direction of the packets) in order to provide the necessary feedback data. In another embodiment of the invention, dedicated physical control channel (DPCCH) bits are used for the fast feedback. The method according to the present invention is described with respect to the downlink transmission of a frequency division duplex (FDD) as an example. Thus the feedback is in the uplink direction. Of course, extension of this description to uplink data is straight-forward, wherein the feedback would be in the downlink direction.
Hybrid ARQ is a link adaption technique which is used to improve the performance of wireless communication systems and the Type 1 Hybrid ARQ with soft combining uses a type of repetition coding in which the retransmitted packet is combined with the initially transmitted packet. Type 2 Hybrid ARQ uses an incremental redundancy scheme and thus both in Type 1 and Type 2 Hybrid ARQ soft combining is utilized in the receiver and soft decisions values of the erroneous packets have to be stored in the receiver which of course, can lead to very high memory requirements at the receiver. It is thus clear that the amount of memory required for storing the soft decisions is proportional to the retransmission time interval.
For wireless communication system, the uplink and downlink radio frames have a probable time displacement between the uplink and downlink channels. For dedicated channels, the uplink and downlink frames are typically separated by 1,024 chips. The purpose of the fast feedback is to reduce the time interval for transmitting the feedback after receiving the packet. However, the receiver must typically de-interleave, de-ratematch, decode and error check the received packets after reception of the radio frame and all of these operations require time. Thus the fastest way to send acknowledgement is to stuff the feedback data in the next frame in the uplink direction so that the transmitter can retransmit the erroneous packets with a delays of only one transmission timing interval (TTI).
The present invention achieves this goal by reserving a few slots, fully or partly, where the number of slots can be a parameter, in the uplink Dedicated Physical Data Channel (DPDCH) radio frame for feedback data only. This technique implies that data in the uplink direction can be transmitted only in the remaining slots (also in the remaining parts of the slots if the slots are only partly used for feedback). The feedback data is transmitted in slots N1 to N2xe2x88x921 and the data in the uplink direction are therefore transmitted in slots 1 to N1xe2x88x921 and in slots N2 to N, where N is the number of slots in a radio frame. In this technique, the value of N1 is dependent upon the time offset between the uplink and downlink channels. It is also dependent upon the time required for any de-interleaving, de-ratematching, decoding and error checking at the receiver. Furthermore, the number of feedback slots (Nfb) depends on the size of the feedback packet. If forward ordering is used, the value of Nfb is typically three or four slots.
An alternative implementation of the fast feedback channel can use some of the dedicated physical control channel (DPCCH) bits in the given slots. Thus feedback bits can be punctured into the pilot, transmit power control (TPC) bits, transport format combination indicator (TFCI) bits and feedback information (FBI) bits of one or several time slots. Alternatively, the feedback information can be time multiplexed with the existing pilot, TPC, TFCI and FBI bits by, for example, changing the spreading factor of the DPCCH so that more channel bits will be available.